target: msm8974: Remove unused panel signature API definition
target: msm8974: Remove unused panel signature API definition

Remove the unused implementation of panel signature read API in 8974
as shared code already has this.

Change-Id: Idfed1de61ff3355ec8a811fcffa1f0e5e198eb40

idor/dk50-bootable_bootloader_lk,mozilla-b2g/fairphone2_kernel_lk,mozilla-b2g/fairphone2_kernel_lk,lg-devs/g2-bootloader,t2m-foxfone/android_kernel_lk,lg-devs/g2-bootloader,idor/dk50-bootable_bootloader_lk,t2m-foxfone/android_kernel_lk,mozilla-b2g/fairphone2_kernel_lk,mozilla-b2g/fairphone2_kernel_lk,t2m-foxfone/android_kernel_lk,idor/dk50-bootable_bootloader_lk,lg-devs/g2-bootloader,idor/dk50-bootable_bootloader_lk,idor/dk50-bootable_bootloader_lk,mozilla-b2g/fairphone2_kernel_lk,t2m-foxfone/android_kernel_lk,lg-devs/g2-bootloader,lg-devs/g2-bootloader,t2m-foxfone/android_kernel_lk